1. Field of the Invention
The present invention relates to a scanning optical system.
2. Description of Related Art
A scanning optical system is essential to the operation of a laser printer, a laser scanner, a bar code reader or the like. In a typical scanning optical system, a polygonal mirror, a hologram disk or the like is used as a light deflector. A laser beam emitted from a semiconductor laser is incident upon, and deflected by, the light deflector. The laser beam subsequently passes through a scanning lens system such as an f.theta. lens system, to scan a predetermined area on a scanning surface (i.e., the main scanning is executed). The scanning surface could be, for example, a sensitive paper or plate. While the main scanning is being executed, the scanning surface is moved in a direction perpendicular to the direction of the main scanning (i.e., the sub-scanning is executed). Thereby, the scanning surface is two-dimensionally scanned.
In order to achieve a high precision or resolution scanning optical system, it is necessary to effect an arrangement so that the f.theta. lens system converges a bundle of rays of a laser beam in such an manner so as to make the diameter of the beam spot on the scanning surface small. In general, the laser beam emitted from a semiconductor laser is converged into an oblong shape extending along the main scanning direction in the vicinity of the light deflector. Moreover, the f.theta. lens system generally has a larger magnification in the sub-scanning direction than in the main-scanning direction due to its function of making the above oblonged-shaped laser beam into a beam spot and focusing it onto the scanning surface. Furthermore, it has been recently confirmed that the magnitude of the spherical aberration in a sub-scanning plane (i.e., a plane extending along the sub-scanning direction) reaches a level that cannot be disregarded when making the diameter of the laser beam incident on the f.theta. lens system large in providing a high precision or resolution scanning optical system.